1. Field of the Invention
The present invention relates to an electrolytic processing apparatus and an electrolytic processing method, and more particularly to an electrolytic processing apparatus and an electrolytic processing method useful for processing a conductive material formed in a surface of a substrate, such as a semiconductor wafer, or for removing impurities adhering to a surface of a substrate.
2. Description of the Related Art
In recent years, instead of using aluminum or aluminum alloys as a material for forming circuits on a substrate such as a semiconductor wafer, there is an eminent movement towards using copper (Cu) which has a low electric resistivity and high electromigration resistance. Copper interconnects are generally formed by filling copper into fine interconnect recesses formed in a surface of a substrate. Various techniques are known for forming such copper interconnects, including chemical vapor deposition (CVD), sputtering, and plating. According to any such technique, a copper film is formed on substantially the entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 1A through 1C illustrate, in sequence of the process steps, an example of forming such a substrate W having copper interconnects. As shown in FIG. 1A, an insulating film 2, such as an oxide film of SiO2 or a film of low-k material, is deposited on a conductive layer 1a in which semiconductor devices are formed, which is formed on a semiconductor base 1. Contact holes 3 and trenches 4 are formed in the insulating film 2 by performing a lithography/etching technique so as to provide interconnect recesses. Thereafter, a barrier layer 5 of TaN or the like is formed on the insulating film 2, and a seed layer 7 as an electric supply layer for electroplating is formed on the barrier layer 5 by sputtering or CVD, or the like.
Then, as shown in FIG. 1B, copper plating is performed onto the surface of the substrate W to fill the contact holes 3 and the trenches 4 with copper and, at the same time, deposit a copper film 6 on the insulating film 2. Thereafter, the copper film 6, the seed layer 7 and the barrier layer 5 on the insulating film 2 are removed by chemical mechanical polishing (CMP) so as to make the surface of the copper film 6 filled in the contact holes 3 and the trenches 4 and the surface of the insulating film 2 lie substantially on the same plane. Interconnects composed of the copper film 6 are thus formed in the insulating film 2, as shown in FIG. 1C.
Components in various types of equipments have recently become finer and have required higher accuracy. As sub-micron manufacturing technology is becoming common, the properties of materials are more and more influenced by the processing method. Under these circumstances, in such a conventional machining method that a desired portion in a workpiece is physically destroyed and removed from the surface thereof by a tool, a large number of defects may be produced to deteriorate the properties of the workpiece. Therefore, it becomes important to perform processing without deteriorating the properties of the materials.
Some processing methods, such as chemical polishing, electrolytic processing, and electrolytic polishing, have been developed in order to solve this problem. In contrast with conventional physical processing, these methods perform removal processing or the like through chemical dissolution reaction. Therefore, these methods do not suffer from defects, such as formation of a damaged layer and dislocation, due to plastic deformation, so that processing can be performed without deteriorating the properties of the materials.
In the case of the above-mentioned conventional electrolytic processing or electrolytic polishing, the process proceeds through an electrochemical interaction between a workpiece and an electrolytic solution (aqueous solution of NaCl, NaNO3, HF, HCl, HNO3, NaOH, etc.). Since an electrolytic solution containing such an electrolyte must be used, contamination of a workpiece with the electrolyte cannot be avoided.
Further, a method has been reported which performs CMP processing simultaneously with plating, viz. chemical mechanical electrolytic polishing. According to this method, the mechanical processing is carried out to the growing surface of a plated film, causing the problem of denaturing of the resulting film.
Electrolytic metal processing methods, which are improved in environmental burden, contamination of a processed product, safety in operation, etc., have recently been developed (see Japanese Patent Laid-Open Publication Nos. 2000-52235 and 2001-64799). These electrolytic processing methods use pure water or ultrapure water in carrying out electrolytic processing. Since pure water or ultrapure water hardly passes electricity therethrough, the processing methods use an ion exchanger disposed between a workpiece and a processing electrode to carry out electrolytic processing of the workpiece. Since the workpiece, the ion exchanger and the processing electrode are all put in a pure water or ultrapure water atmosphere, the environmental burden problem and the workpiece contamination problem can be remarkably reduced. Further, metal to be processed is removed as metal ions through the electrolytic reaction, and the dissolved ions are held in the ion exchanger. This can further reduce contamination of the workpiece and the liquid (pure water or ultrapure water) itself. Such a processing method, therefore, is considered to be an ideal electrolytic processing method.
As described above, according to the electrolytic processing method which processes a workpiece by using an ion exchanger and supplying ultrapure water, contamination of the workpiece can be prevented and the environmental burden can be remarkably reduced. Further, the electrolytic processing method can provide various metal parts with a specular gloss surface, and can also eliminate the use of a cutting oil, a slurry containing a polishing agent, an electrolyte solution, etc. which are necessary to the conventional mechanical metal processing or finishing methods.
Chemical mechanical polishing (CMP), for example, generally necessitates a complicated operation and control, and needs a considerably long processing time. In addition, a sufficient cleaning of a polished surface must be conducted after the polishing treatment. This also imposes a considerable load on the slurry or cleaning liquid waste disposal. Accordingly, there is a strong demand for omitting CMP entirely or reducing a load upon CMP. Also in this connection, it is to be pointed out that though a low-k material, which has a low dielectric constant, is expected to be predominantly used in the future as a material for the insulating film (interlevel dielectric layer), the low-k material has a low mechanical strength and therefore it is difficult for it to endure the stress applied during CMP processing. Thus, also from this standpoint, there is a demand for a process that enables the flattening of a substrate without giving any stress thereto.
In order to solve these problems, a method has been proposed in which electrolytic processing of a workpiece is carried out by using a liquid having a high electric resistance, such as pure water or ultrapure water, as an electrolyte solution and providing, according to necessity, an ion exchanger, which promotes the dissociation reaction of water molecules in the liquid into hydroxide ions and hydrogen ions, between an electrode and the workpiece, thereby eliminating a mechanical stress on the workpiece and facilitating post-cleaning (see Japanese Patent Laid-Open Publication No. 2003-145354).
There are also proposed a method in which electrolytic processing (etching) of a workpiece with an electrolytic solution is carried out while keeping the distance between the workpiece and an opposing electrode not more than 10 μm, for example 1 μm, thereby improving the surface flatness of the workpiece after processing (see Japanese Patent Laid-Open Publication No. 2001-102356), and a method in which electrolytic processing of a wafer (substrate) with an electrolytic solution is carried out while keeping the distance between a platen electrode and the wafer not more than 1 mm, preferably 2000 Å and keeping a polishing pad (thickness of about 200 Å), interposed between the platen electrode and the wafer, in contact with the wafer surface to polish the wafer surface (WO No. 02/064314).
In electrolytic processing using an ion exchanger, the ion exchanger is fixedly disposed between a workpiece and at least one of a processing electrode and a feeding electrode, thereby promoting the dissociation reaction of water and supplying OH− ions or H+ ions generated to the surface of the workpiece to effect processing thereof. Ion exchangers in different forms have different degrees of water permeability. When an ion exchanger having poor water permeability is used, for example, the supply of water to between a workpiece and an electrode (processing electrode or feeding electrode) is restricted. The supply of water to the ion exchanger may therefore be insufficient, resulting in a low water decomposition efficiency and thus an insufficient supply of OH− ions or H+ ions as reaction species. Further, the efficiency of removal of processing products and gasses generated at the surfaces of the workpiece and the electrode during processing will be low. This may result in deterioration of the morphology of the processed surface.
A processing product, which will affect the processing, gradually accumulates in an ion exchanger with the progress of processing. A method to solve this problem is to electrically remove the processing product accumulated in the ion exchanger. This method, however, is not a permanent solution yet. The ion exchanger, therefore, needs to be finally changed for a new one. During the change of the ion exchanger, the apparatus must be stopped, leading to a lowering of the throughput.
There is, therefore, a demand for the development of an electrolytic processing apparatus and method that satisfies the requirements of (1) an increased amount of water supply to an ion exchanger, (2) efficient removal of processing products and gasses generated upon processing and (3) ease of a change of ion exchanger.
In electrolytic processing using an electrolyte solution to remove an unnecessary portion of a conductive film having surface irregularities, for example, the copper film 6 embedded in trenches shown in FIG. 1B, formed on a substrate such as a semiconductor wafer, difficulty in flattening the surface irregularities of the conductive film is a typical problem involved in such processing. This difficulty is ascribable to the fact that an appropriate difference in processing rate between recessed portions and raised portions of the surface irregularities of a conductive film formed on a substrate cannot be produced due to (1) the distance between the substrate and an electrode being much larger than the surface level difference of the conductive film formed on the substrate and (2) low electric resistivity of electrolyte solution. Further, in the case of interposing an ion exchanger between a workpiece, especially a substrate, and an electrode, electrolytic processing is generally carried out while sliding the ion exchanger on the surface of the workpiece. The sliding movement between the ion exchanger and the workpiece can produce defects in the surface of the workpiece and wear the ion exchanger. Solving these problems is therefore required in applying electrolytic processing to removal of a conductive film provided on a substrate, such as a semiconductor wafer.